Atherosclerosis is a chronic inflammatory disease and is the leading cause of morbidity and death in developed countries. However, there are still fundamental gaps in our knowledge of the underlying mechanisms that contribute to its development, and end-stage clinical events including plaque rupture with possible myocardial infarction or stroke. Although the dogma in the field is that an increased ratio of smooth muscle cells (SMC) to macrophages within lesions promotes plaque stability, there are major limitations in the experimental evidence for this model including major ambiguities regarding which cells within lesions are of SMC versus monocyte-macrophage origin and the mechanisms that control SMC and macrophage number and phenotype. Of major significance, studies employing unique SMC lineage tracing ApoE-/- mice developed by the Owens lab provide evidence that ~25% of cells within advanced lesions that are Mac2+ but negative for SMC markers including SM ?-actin (SM?A), are SMC- rather than macrophage-derived. Conversely, a significant fraction of SM?A+ fibrous cap cells, presumed to be SMC-derived, are not. Moreover, we have evidence in that a significant fraction of macrophage- derived cells show reduced macrophage marker expression but are SM?A+. Collectively, results show that macrophages may convert to SMC-like cells and SMC to macrophage-like cells. Moreover, results indicate that it is likely that many lesion cells have been mis-identified in previous studies in the field, thus greatly confounding our understanding of the mechanisms and factors that regulate phenotypic transitions of these cells. The central focus of this grant is to determine the effects of genetic or pharmacological inhibition of IL1? and IL1R1 signaling on phenotypic transitions of SMC and macrophages, as well as on the overall size and stability of late stage atherosclerotic lesions. Whereas there is good evidence that disruption of IL1? signaling inhibits formation of fatty streaks and early stage lesions, the role of IL1 in late stage lesions is unclear. Aim 1a will use novel utilize SMC and myeloid specific lineage tracing IL1R1 knockout mouse lines generated by our lab to test the hypothesis that IL1R1-dependent transitions in phenotype of SMC and macrophages within advanced atherosclerotic lesions play a critical role in determining overall plaque and lumen size, as well as lesion composition including multiple indices of plaque stability. Aim 1b will be o determine if phenotypic transitions observed in our mouse studies occur in human atherosclerotic lesions based on analysis of autopsy specimens using a highly novel epigenetic SMC lineage tracing method recently developed by our laboratory (Gomez et al., Nature Methods). Aim 2 will determine if treatment of ApoE null mice with a Novartis mouse anti-IL1? antibody induces changes in lesion size, cellular composition, or indices of plaque stability, as well as transitions in SMC and/or macrophage phenotype.